1. Field of the Invention
The present invention relates to a radio frequency switch usable in an antenna for transmission and reception switching in a wireless communication apparatus, and a wireless communication apparatus using the same.
2. Description of the Related Art
In a system for transmitting and receiving in time division, generally, the antenna is composed by using a switch. Examples of system of communication by time division include PHS (Personal Handyphone System) of Japan and key-less entry system of an automobile.
A conventional circuit configuration of an antenna using a switch is shown in FIG. 2. As shown in FIG. 2, the conventional circuit comprises transmission terminal 101, an antenna terminal 102, a reception terminal 103, a control terminal 104, a transmission power amplifier 105, capacitors 106 to 108, an inductor 109, a current control resistance 110, a bypass capacitor 111, a choke coil 112 for cutting off radio frequency, diodes 113, 115, a quarter wavelength transmission line 114 at transmission frequency, a band pass filter 116 such as SAW filter, a low noise amplifier (LNA) 117 for amplifying reception signal, and a mixer 118 for converting frequency to intermediate frequency.
In this configuration, the capacitors 106 to 108 and inductor 109 compose a low pass filter. In particular, a parallel circuit of the capacitor 108 and inductor 109 resonates at a frequency of about 2 times of the transmission frequency, and removes the harmonic components of the output signal of the power amplifier 105. Further, when a current flows from the control terminal 104 into the diodes 113, 115 and quarter wavelength transmission line 114, the antenna terminal 102 and transmission terminal 101 are connected, and when the current is cut off, the antenna terminal 102 and reception terminal 103 are connected.
The band pass filter 116 removes the undesired signal outside of the band, and prevents occurrence of distortion in the LNA 117. The band pass filter 116 also removes image frequency component which becomes noise in the intermediate frequency component generated in the mixer 118. For the convenience of image removing characteristic, the SAW filter is often used.
In this configuration, however, especially when receiving, the reception sensitivity deteriorates due to insertion loss of the quarter wavelength transmission line 114 and band pass filter 116. When using the quarter wavelength transmission line 114 and SAW filter, the insertion loss of the band pass filter 116 is respectively about 0.5 dB and 3.5 dB, and the total loss may reach as high as about 4.0 dB.
Therefore, in order to enhance the sensitivity further, it is required to decrease the insertion loss in the reception route from the antenna before input to the LNA, in particular.